Detectors for infrared radiation or visible radiation typically include a substrate having on one side thereof a two-dimensional array of pixel detectors, arranged in rows and columns. In certain applications, it is desirable to polarize the radiation traveling to the pixel detectors. For this purpose, a micropolarizing filter can be provided over the array of pixel detectors.
A typical micropolarizing filter includes a rigid substrate having a thickness of approximately 600 microns, and a metal layer on one side of the substrate. The substrate is made of a material through which the radiation of interest can pass. For example, in the case of infrared radiation, the substrate can be gallium arsenide (GaAs). The metal layer can be any one of a number of different types of metal. The metal layer is typically a thin layer of gold, with a thickness of approximately 1 micron, and with a plurality of groups of parallel slots extending through it. Each group of slots corresponds to and is aligned with a respective pixel detector, and effects polarization of radiation traveling to that pixel detector. While pre-existing filters of this type have been generally adequate for their intended purposes, they have not been satisfactory in all respects.
For example, visible radiation and infrared radiation have wavelengths falling in a range from approximately 1 micron to 12 microns. Since the substrate is many times thicker than a wavelength of the radiation of interest, the substrate can produce undesirable optical effects. For example, the substrate typically causes a degree of scattering and/or defocusing of the radiation, such that a portion of the radiation traveling toward any given pixel detector ends up being misdirected toward one or more neighboring pixel detectors. As a result, there is a degree of optical cross-talk between the detectors. The large thickness of the substrate can affect the focal point of the detector front lens, and can also cause a significant degree of optical loss within the filter, such that the filter is not very efficient.
Another consideration is that the substrate has a relatively high index of refraction. Consequently, in order to achieve an effective degree of polarization, which is commonly known as a high extinction ratio, an anti-reflection (AR) coating is needed on each side of the substrate, and the pitch between the adjacent slots in the metal layer needs to be relatively small. In order to achieve a small pitch, fine lithography is needed for fabrication of the metal layer, which in turn increases the fabrication cost.
A further disadvantage of the relatively thick GaAS substrate is that it has limited optical transparency. Therefore, when the filter is being installed on a detector array, the detectors cannot easily be seen through the filter, and it is difficult to achieve accurate alignment between the filter and the detectors. In fact, it is fairly common to incur the cost and effort of using a laser to ablate alignment holes in the substrate, in order to be able to visually see through it for purposes of achieving accurate alignment with the substrate.